Peristaltic pumps are devices used to pump fluids contained in compressible tubes. The fluids are moved through the tubing by compressing the tubing with a roller or similar device and moving the area of compression along the length of the tube to force the fluid through the tube. In a rotary peristaltic pump, the rollers or other compression devices are typically contained on a rotor that is rotated to provide the desired movement.
One advantage of peristaltic pumps is their relatively simple design. Another advantage is that contamination of the fluids being pumped is not an issue when using peristaltic pumps because the fluid being pumped does not come into contact with anything but the tubing (which in many applications is disposable). Peristaltic pumps are also relatively insensitive to variations in the viscosity of the fluids being pumped as well as pressure fluctuations in the system. The use of peristaltic pumps may also reduce or eliminate the need for isolation valves as the compression of the tube can isolate the upstream fluid from the downstream fluid.
Although peristaltic pumps provide a number of advantages, they do suffer from some drawbacks. One disadvantage is that the fluid flow from peristaltic pumps is typically characterized by pulsations as the tubing is compressed and uncompressed during pumping. Furthermore, highly accurate flow rate may be difficult to achieve due to the pulsating flow. Another factor that may contribute to pulsations is that, in rotary peristaltic pumps incorporating compression devices on a rotor, highly accurate control of the rotor is difficult to attain due to the torque variations as the compression devices alternately compress the tubing and then release the tubing. Those torque fluctuations, combined with the pulsations caused by compression and decompression of the tubing, make highly accurate control over the fluid flow rate from a rotary peristaltic pump difficult to achieve.
Attempts at controlling pulsation and flow rate when using peristaltic pumps have included the use of controllers to vary the speed of the compression devices, using parallel peristaltic pumps in which the fluid flow pulses are offset in a balanced manner, and using additional devices downstream from the pump to counteract the pulsations produced by the pump.
None of these approaches have, however, provided the desired flow rate control in conjunction with control over pressure variations, especially in applications where flow rates are relatively low and the need for accuracy is high. One such application is in the delivery of semiconductor lithography process fluids including photoresists, solvents, developers, water, etc. These fluids are typically applied in carefully controlled amounts during the construction of an integrated circuit on a semiconductor wafer. Typical flow rates required in these applications range from about 0.1 to about 100 cubic centimeters per second and it is highly desirable that pulsations in the fluid flow be controlled during delivery.
As a result, a need exists for a rotary peristaltic pump that can supply fluid accurately at desired flow rates and with the desired control over pulsations normally experienced when using peristaltic pumps.